Use only regular chalk to write on the silkscreened chalkboard area. Erase with a damp cloth before washing. Do not use chalkboard markers. They will be permanent, but then fade a lot in the wash (which does not look good).

TIP: to make the chalk brighter and stick better, get your chalk a little wet before you write on your shirt. It may look dull as it goes on, but it will dry nice and bright.